The present invention relates generally to a color calibration method for imaging color measurement devices. More particularly, the present invention relates to a calibrating means for a color measurement system for the measurement of color, in quantifiable units.
It is often desired to know the color of light emitted by an illumination system, a light source, or a display system in order to characterize its performance. Traditionally, this has been accomplished by a calorimeter or a spectroradiometer, which measures the average color over an angular extent.
Currently, the spectroradiometer is the most accurate instrument for determining the color of light in quantifiable units. The spectroradiometer captures the entire spectrum of a visible light emitted by a light source. The light source includes any surface or volume that emits, transmits or reflects optical radiation. The measured spectral data may be mathematically analyzed to determine the color of the light defined by a standard color space. Commonly used color spaces for illumination systems are CIE (Commission Internationale de l'Eclairage) 1931 (x,y) chromaticity coordinates, CIE 1964 (u′,v′) chromaticity coordinates, and the L*, a*, b* color space. In principle, an infinite number of color space are possible. However, only a few are routinely used in industrial practice.
A diagram of the experimental set-up for display measurements made with a spectroradiometer are shown in FIG. 1. The spectroradiometer 12 captures light from some small portion of the display surface 10 (the light is emitted in the direction of the spectroradiometer) and acquires data of the spectral content of the light. A sample of four spectroradiometer measurement data sets is shown in FIG. 2. The vertical axis of the graph is normally recorded in W/nm/sr. The horizontal axis of the graph shows the wavelength of light, commonly in nm. The spectral power distribution of light is commonly written as function S(λ). The spectroradiometer is able to calculate the CIE 1931 tristimulus values by employing the following formulas defined by CIE:       X    =                  ∫                  380          ⁢                                          ⁢          nm                          780          ⁢                                          ⁢          nm                    ⁢                        S          ⁡                      (            λ            )                          ⁢                  x          ⁡                      (            λ            )                          ⁢                  ⅆ          λ                      ,      Y    =                  ∫                  380          ⁢                                          ⁢          nm                          780          ⁢                                          ⁢          nm                    ⁢                        S          ⁡                      (            λ            )                          ⁢                  y          ⁡                      (            λ            )                          ⁢                  ⅆ          λ                      ,          ⁢      Z    =                  ∫                  380          ⁢                                          ⁢          nm                          780          ⁢                                          ⁢          nm                    ⁢                        S          ⁡                      (            λ            )                          ⁢                  z          ⁡                      (            λ            )                          ⁢                  ⅆ          λ                    In the above formulas, x(λ), y(λ) and z(λ) are the CIE 1931 2 degree standard observer color matching functions specifically defined by the CIE to serve as international definitions for the CIE 1931 (x,y) color space. The chromaticity coordinates are then determined by the following formulas:       x    =          X              X        +        Y        +        Z              ,      y    =          Y              X        +        Y        +        Z            
If the spectroradiometer has been calibrated to accurately measure the spectral content of light, there is no further calibration required of a spectroradiometer to accurately measure the CIE 1931 tristimulus values and calculate the 1931 chromaticity coordinates accurately.
A calorimeter is often used as a color measurement device instead of a spectroradiometer because the cost of a calorimeter is substantially less than a spectroradiometer. Most calorimeters are tristimulus calorimeters, which means that they attempt to measure the X, Y and Z CIE 1931 chromaticity matching functions. A calorimeter normally consists of a lens to focus the light onto a detector (sometimes multiple detectors) and color filters disposed between the lens and the detector(s). The spectral response of the traditional tristimulus calorimeter closely approximates the x(λ), y(λ), and z(λ) tristimulus matching functions shown as FIG. 3. As shown in FIG. 3, the spectral response of a real calorimeter system is not an exact match to the spectral curves defined by the CIE 1931 color space.
In order to account for this spectral mismatch, the prior art calorimeters must be calibrated to measure the color more accurately. In general, both the magnitude and the shape of the spectral response curves do not match the CIE 1931 tristimulus values. The most common means of performing a calorimeter calibration has been to use a single point color calibration technique. During the calibration procedure, the calorimeter first measures uncorrected tristimulus values (Xuncorrected, Yuncorrected, Zuncorrected) for the spectrum of the standard light source, SIlluminant A(λ) Illuminant A defined by CIE is the most commonly used light source spectrum for calibration of calorimeter. Since the spectrum of the standard light source SIlluminant A(λ) is known, the chromaticity coordinates of the standard light source can be calculated. The chromaticity coordinates are designated as (xIlluminantA, yIlluminantA). Given these values, the calibration coefficients of the calorimeter (k1, k2, k3) can be calculated by the following equations.                               x          corrected                =                                            k1              *                        ⁢                          X              uncorrected                                                                          k1                *                            ⁢                              X                uncorrected                                      +                                          k2                *                            ⁢                              Y                uncorrected                                      +                                          k3                *                            ⁢                              Z                uncorrected                                                                                      y          corrected                =                                            k2              *                        ⁢                          Y              uncorrected                                                                          k1                *                            ⁢                              X                uncorrected                                      +                                          k2                *                            ⁢                              Y                uncorrected                                      +                                          k3                *                            ⁢                              Z                uncorrected                                                                                                                k1              *                        ⁢                          X              uncorrected                                +                                    k2              *                        ⁢                          Y              uncorrected                                +                                    k3              *                        ⁢                          Z              uncorrected                                      =        1                        where                                    X          corrected                =                                            k1              *                        ⁢                          X              uncorrected                                =                      k1            ⁢                                          ∫                                  380                  ⁢                                                                          ⁢                  nm                                                  780                  ⁢                                                                          ⁢                  nm                                            ⁢                                                                                          S                                              illuminant                        ⁢                                                                                                  ⁢                        A                                                              ⁡                                          (                      λ                      )                                                        ·                                                            x                      colorimeter                                        ⁡                                          (                      λ                      )                                                                      ⁢                                  ⅆ                  λ                                                                                                      Y          corrected                =                                            k2              *                        ⁢                          Y              uncorrected                                =                      k2            ⁢                                          ∫                                  380                  ⁢                                                                          ⁢                  nm                                                  780                  ⁢                                                                          ⁢                  nm                                            ⁢                                                                                          S                                              illuminant                        ⁢                                                                                                  ⁢                        A                                                              ⁡                                          (                      λ                      )                                                        ·                                                            y                      colorimeter                                        ⁡                                          (                      λ                      )                                                                      ⁢                                  ⅆ                  λ                                                                                                      Z          corrected                =                                            k3              *                        ⁢                          Z              uncorrected                                =                      k3            ⁢                                          ∫                                  380                  ⁢                                                                          ⁢                  nm                                                  780                  ⁢                                                                          ⁢                  nm                                            ⁢                                                                                          S                                              illuminant                        ⁢                                                                                                  ⁢                        A                                                              ⁡                                          (                      λ                      )                                                        ·                                                            z                      colorimeter                                        ⁡                                          (                      λ                      )                                                                      ⁢                                  ⅆ                  λ                                                                        By storing the values k1, k2 and k3 and using them to calculate Xcorrected, Ycorrected and Zcorrected, the colorimeter is able to make accurate measurement of the chromaticity coordinates for calibration light source.
In FIG. 1, the measurements made with a spectroradiometer or a colorimeter extend over only a small area of the display device under test. Therefore, the traditional measurement comprises an average color or luminance measurement taken over a small portion of the light emitting area of the device under test. When a color measurement is performed on a display at multiple test locations, one has to reposition the prior art colorimeter or spectroradiometer from one point to another to measure the color of light at each test location.